With the advance of constructions of large-volume optical transmission networks, there are problems to be addressed such as how to further expand transmission capacities and extend transmission distances. Instead of an optical intensity modulation scheme for transmitting data using ON/OFF of light, an optical transmission network using an optical phase modulation scheme such as DPSK (Differential Phase Shift Keying) is under study as one means for solving the problems. For example, FIG. 10 illustrates an optical transmitter based on a DPSK modulation scheme used for an optical transmission network using such a phase modulation scheme.
The optical transmitter illustrated in FIG. 10 has a configuration in which CW (Continuous Wave) light output from a semiconductor laser 1 is phase-modulated through a Mach-Zehnder type optical phase modulator 2 and transmitted as an optical signal. The optical phase modulator 2 outputs a phase-modulated optical signal using a drive signal output from a driver 4 according to a data signal that has passed through a DPSK precoder 3, and the mechanism thereof is shown in FIG. 11. A Mach-Zehnder type optical waveguide 21 is formed in an electro-optical crystal substrate 20 represented by LiNbO3 (hereinafter referred to as “LN”) and traveling wave electrodes 22 and 23 are formed on the surface of the substrate 20. The optical waveguide 21 is a Mach-Zehnder type optical waveguide including an input section that faces a substrate end face for inputting light, branch modulation sections (arms) that branch and extend from the input section and an output section that combines the branch modulation sections and faces a substrate end face. When electric fields from the traveling wave electrodes 22 and 23 act on the parts of the branch modulation sections of the optical waveguide 21, the refractive index of the optical waveguide changes and phase modulation takes place.
As for the voltages of the drive signals applied to the electrodes 22 and 23, when a voltage that causes the phase difference between light beams propagating through the two branch modulation sections of the optical waveguide 21 to be a half wavelength is Vπ, giving a voltage Vπ to one electrode 22 and a voltage −Vπ to the other electrode 23 (inverted) allows the output light to have a phase variation of π. Therefore, the drive signal output from the driver 4 causes the one electrode 22 to vary from 0 to Vπ and causes the other electrode 23 to vary from 0 to −Vπ, and can thereby perform optical phase modulation of phase 0, π (e.g., non-Patent Document “40-Gb/s RZ-Differential Phase Shift Keyed Transmission”, A. Gnauck, ThE1, pp. 450-451, Vol. 2, OFC 2003).
For the optical phase modulator 20 based on the above described scheme, electric signals of opposite phases need to be simultaneously applied to the electrodes 22 and 23 of both branch modulation sections, and therefore there is a problem that it is difficult to adjust timing of drive signals in high-speed transmission of 40 Gb/s or the like. Thus, instead of such a dual drive scheme, there is also a proposal of a single drive scheme having one traveling wave electrode to which a drive signal is applied, such as an optical phase modulator using an X-cut LN substrate as shown in FIG. 12A or an optical phase modulator with a polarization inversion region formed in a Z-cut LN substrate as shown in FIG. 12B.
However, when this single drive scheme is adopted, if the voltage that causes the optical phase difference between the branch modulation sections to be a half optical wavelength is assumed to be Vπ as described above, the drive signal needs to be made to vary from 0 to 2Vπ to realize optical phase modulation of phase 0, π. That is, under the single drive scheme, the voltage of the drive signal increases, which is a problem to be solved when applied to high-speed transmission.